High herkogamy but low reciprocity characterizes isoplethic populations of Jasminum malabaricum,a species with stigma‐height dimorphism

• Studies of floral polymorphisms have focused on heterostyly, while stigma‐height dimorphism has received considerably less attention. Few studies have examined the reproductive biology of species with stigma‐height dimorphism to understand how factors influencing mate availability and pollen transfer are related to morph ratios in populations.
• Floral morphological traits, especially herkogamy and reciprocity, pollinator visitation, breeding system and spatiotemporal mate availability, are known to affect inter‐morph pollination and morph ratios in species with stigma‐height dimorphism. In this study, we investigated the presence of stigma‐height dimorphism and estimated morph ratios in four naturally occurring populations of Jasminum malabaricum. We quantified morph‐ and population‐specific differences in the abovementioned factors in these populations to understand the observed morph ratios.
• The positions of anthers and stigmas were characteristic of stigma‐height dimorphism, the first report of this polymorphism in the genus. All study populations were isoplethic, implying equal fitness of both morphs. Herkogamy was higher in the short‐styled morph, while reciprocity was higher between the long‐styled stigma and short‐styled anthers. Long‐ and short‐tongued pollinators were common floral visitors, and we observed no differences between morphs in spatiotemporal mate availability or pollinator visitation. Neither morph exhibited self‐ or heteromorphic incompatibility.
• The short‐styled stigma had lower reciprocity but likely receives sufficient inter‐morph pollen from long‐tongued pollinators, and also by avoiding self‐pollination due to higher herkogamy. These results highlight the importance of sufficient effective pollinators and floral morphological features, particularly herkogamy, in maintaining isoplethy in species with stigma‐height dimorphism.

     

Disassortative mating prevails in style‐dimorphic Narcissus papyraceus despite low reciprocity and compatibility of morphs

Evolution to reduce inbreeding can favor disassortative (intermorph) over assortative (intramorph) mating in hermaphroditic sexually polymorphic plant species. Heterostyly enhances disassortative pollination through reciprocal placement of stigmas and anthers of morphs and appropriate pollinators. Stylar dimorphism in which there is not reciprocal anther placement may compromise disassortative mating, particularly when there is not intramorph incompatibility. Variable rates of disassortative mating along with differential female fecundity or siring success among floral morphs could lead to variation in morph ratio. We investigated mating patterns, female fecundity, and siring success of style‐length morphs in Narcissus papyraceus, a self‐incompatible but morph‐compatible species with dimorphic (long‐ and short‐styled) and monomorphic (long‐styled) populations in central and north regions of its range, respectively. We established experimental populations in both regions and exposed them to ambient pollinators. Using paternity analysis, we found similar siring success of morphs and high disassortative mating in most populations. Female fecundity of morphs was similar in all populations. Although these results could not completely explain the loss of dimorphism in the species’ northern range, they provided evidence for the evolutionary stability of stylar dimorphism in N. papyraceus in at least some populations. Our findings support the hypothesis that prevailing intermorph mating is key for the maintenance of stylar dimorphism.     

Stylar polymorphism, reciprocity and incompatibility systems in Nymphoides montana (Menyanthaceae) endemic to southeastern Australia

Heterostyly is a stylar polymorphism that has been shaped by the evolution of floral characters adapted for efficient pollen transfer. Different types of stylar polymorphism are described in which the discrete characterization of the exact polymorphic type (e.g., distyly vs. stigma-height dimorphism) requires detailed floral measurements (e.g., sex-organ reciprocity). In clonal and aquatic Nymphoides montana, although the presence of two floral morphs that contain styles of two lengths has been previously reported, no studies have quantitatively estimated the level of reciprocity and/or described the stylar condition. Morphological variations and incompatibility relationships were explored between the two morphs in three southeastern Australian populations. In this study, one population is characterized as stigma-height dimorphism (i.e., two morphs with discrete variation in stigma height but little variation in anther height), whereas the other two populations are typical distylous (i.e., two morphs for reciprocal stigma and anther height). Nymphoides montana is dimorphic in a wide range of ancillary characters, including corolla size, stigma size, shape and papillae morphology, and pollen size, number and exine sculpture. Following glasshouse pollinations, full incompatibility systems were observed in the distylous populations, whereas the stigma-height dimorphic population showed between-morph variation in the extent of incompatibility. Despite the variation in sex-organ reciprocity and incompatibility, other lines of evidence appear to assure the maintenance of the stylar polymorphism in the N. montana study populations. All populations are nearly isoplethic (i.e., both morphs in equal frequencies), which is indicative of balanced polymorphisms that appear to be maintained by legitimate pollen transfer between the morphs.     

Pollinator shifts and the loss of style polymorphism in Narcissus papyraceus (Amaryllidaceae)

Darwin proposed that the driving force for the evolution of style polymorphisms is the promotion of cross‐pollination between style morphs, through accurate placement of pollen on the pollinator’s body. This hypothesis has received much attention, but the effect of different pollinators in the fitness of morphs remains poorly understood. Narcissus papyraceus is a style dimorphic species (long ‐L‐ and short ‐S‐ styled) with isoplethic (1 : 1) and L‐monomorphic populations, mainly visited by long‐tongued (LT) nocturnal and short‐tongued (ST) diurnal pollinators, respectively. We studied natural female fertility of morphs, and assessed the role of diurnal and nocturnal pollinators. We also quantified female fertility of the morphs in experimental populations with different morph ratio, exposed to predominately long‐ or short‐tongued pollinators. We found that with LT pollinators, both morphs were successfully pollinated in all morph ratio conditions, suggesting that these insects could be involved in maintenance of the polymorphism, although other factors may also play a role. However, with ST pollinators, S‐plants displayed less fertility than L‐plants, and mating among L‐plants was favoured, implying that the polymorphism is lost. These results underscore the role of pollinators on variations in style polymorphism.     

Parallel chemical switches underlying pollinator isolation in Asian Mitella

Floral scents are among the key signals used by pollinators to navigate to specific flowers. Thus, evolutionary changes in scents should have strong impacts on plant diversification, although scent‐mediated plant speciation through pollinator shifts has rarely been demonstrated, despite being likely. To examine whether and how scent‐mediated plant speciation may have occurred, we investigated the Asimitellaria plant lineage using multidisciplinary approaches including pollinator observations, chemical analyses of the floral scents, electroantennographic analyses and behavioural bioassays with the pollinators. We also performed phylogenetically independent contrast analyses of the pollinator/floral scent associations. First, we confirmed that the pairs of the sympatric, cross‐fertile Asimitellaria species in three study sites consistently attract different pollinators, namely long‐tongued and short‐tongued fungus gnats. We also found that a stereoisomeric set of floral volatiles, the lilac aldehydes, could be responsible for the pollinator specificity. This is because the compounds consistently elicited responses in the antennae of the long‐tongued fungus gnats and had contrasting effects on the two pollinators, that is triggering the nectaring behaviour of long‐tongued fungus gnats while repelling short‐tongued fungus gnats in a laboratory experiment. Moreover, we discovered that volatile composition repeatedly switched in Asimitellaria between species adapted to long‐tongued and short‐tongued fungus gnats. Collectively, our results support the idea that recurrent scent‐mediated speciation has taken place in the Asimitellaria–fungus gnat system.     

The evolution of polymorphic sexual systems in daffodils (Narcissus)

Narcissus, the daffodil genus, exhibits an unusual diversity of sexual systems, with populations that are monomorphic, dimorphic or trimorphic for style length. Associated with this variation are striking differences among species in floral morphology and pollination biology. This diversity provides an opportunity to investigate the evolution of mating polymorphisms, and to determine how floral morphology promotes transitions among sexual systems. Because of the absence of heteromorphic incompatibility in Narcissus, floral morphology plays a key role in governing patterns of outcrossed mating. Phylogenetic evidence indicates that stylar monomorphism is ancestral in the genus, with multiple origins of stylar polymorphism, including independent origins of stigma-height dimorphism, distyly and tristyly. Sexual polymorphisms have evolved only in lineages with narrow floral tubes that are pollinated by Lepidoptera and/or long-tongued bees. Populations of polymorphic Narcissus species are typically dominated by the long-styled morph and display imperfect reciprocity in the positions of sexual organs. These features are consequences of the unusual association between stylar polymorphism and a self-incompatibility system that permits intramorph mating.     

New evidence for the Darwinian hypothesis of heterostyly: breeding systems and pollinators in Narcissus sect. Apodanthi

Here we analysed the role played by breeding systems and pollinators in the evolution of heterostyly by testing whether evolution towards heterostyly is associated with style polymorphism and changes in pollinator proficiency or breeding system variation (Darwinian hypothesis). We studied pollinators, pollen-transfer efficiency, and incompatibility systems in all seven species of Narcissus sect. Apodanthi for which we also obtained chloroplast DNA (cpDNA) sequences from three spacers to infer phylogenetic relationships. Five species are self-incompatible and within-morph cross-compatible. Heterostylous (Narcissus albimarginatus) and style-dimorphic (Narcissus cuatrecasasii) species that have a high degree of reciprocity in stigma and anther height are primarily pollinated by solitary bees. The style-monomorphic species (Narcissus watieri) and the style-dimorphic species with the least stigma-anther reciprocity (Narcissus rupicola) are both self-compatible and pollinated by butterflies, moths and hover flies. Phylogenetic reconstruction of character transitions indicates that the shift from style dimorphism to distyly is associated with a shift to bee pollination. Pollination by lepidopterans and flies is associated with stable style dimorphism and monomorphism. Evolution and maintenance of style polymorphisms in this group of species are independent of incompatibility systems. Taken together, our results strongly support the pollinator-based model for evolution of heterostyly and style length polymorphisms in general.     

Self‐incompatibility and post‐fertilization maternal regulation cause low fecundity in Aegle marmelos (Rutaceae)

Outbreeding confers an evolutionary advantage, and flowering plants have evolved a variety of contrivances for its maximization. However, neither fruit set nor seed set is realized to its fullest potential for a variety of reasons. The causes of low flower to fruit and seed to ovule ratios were investigated in naturally occurring bael trees (Aegle marmelos) at two sites for three seasons. The study established that the mass effect of synchronized flowering attracted a variety of insect pollinators to the generalist flowers; Apis dorsata was the most efficient pollinator. The seed to ovule ratio was low despite high natural pollination efficiency (c. 2400 pollen per stigma). Although pollination‐induced structural and histochemical changes in the style allowed many (9.5 ± 2.1) pollen tubes to grow, only cross‐pollen tubes could grow through the style. Gametophytic self‐incompatibility, manifested in the stylar zone, resulted in a significantly slower growth rate of self‐pollen tubes. The occurrence of obligate self‐incompatibility, coupled with increased self‐pollen deposition (geitonogamy), caused a significant number of flowers to abort. Fruit retention in the trees declined from 40% to 12% as a result of abortion of fruits at different stages of development. The number of mature fruits on a tree was negatively correlated (r = ?0.82) with their size. It is inferred that low natural fecundity in A. marmelos is primarily a result of obligate self‐incompatibility and strong post‐fertilization maternal regulation of allocation of resources to the developing fruits. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172 , 572–585.     

Interactions between hawkmoths and flowering plants in East Africa: polyphagy and evolutionary specialization in an ecological context

Hawkmoths (Lepidoptera, Sphingidae) are considered important pollinators in tropical regions, but the frequency and degree of reciprocal specialization of interactions between hawkmoths and flowers remain poorly understood. Detailed observations at two sites in Kenya over a two‐year period indicate that adult hawkmoths are routinely polyphagous and opportunistic, regardless of their proboscis length. About 700 individuals of 13 hawkmoth species were observed visiting a wide range of plant species at the study sites, including 25 taxa that appear to be specifically adapted for pollination by hawkmoths. We estimate that 277 plant species in Kenya (c. 4.61% of the total angiosperm flora) are adapted for pollination by hawkmoths. Floral tube lengths of these plants have a bimodal distribution, reflecting the existence of two hawkmoth guilds differing in tongue length. Hawkmoths exhibited strongly crepuscular foraging patterns with activity confined to a 20‐min period at dusk and, in some cases, a similar period just before dawn. Corolla tube length appears to act as a mechanical filter as the longest‐tubed plants were visited by the fewest hawkmoth species and these were exclusively from the long‐tongued guild. Tube length showed a strong positive relationship with nectar volume, even after phylogenetic correction, which implies that plants with long corolla tubes are under selection to offer relatively large amounts of nectar to entice visits by polyphagous long‐tongued hawkmoths. Our study shows that diffusely co‐evolved pollination systems involving long‐tongued hawkmoths are clearly asymmetrical, with plants exhibiting a high degree of floral specialization, while hawkmoths exhibit polyphagous behaviour. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 199–213.     

Darwin's cross-promotion hypothesis and the evolution of stylar polymorphism

There is growing appreciation that the ecological factors which impact on rates of pollen transfer can contribute significantly to reproductive trait evolution in plants. In heterostylous species, several studies support Darwin's claim that the reciprocal positions of stigmas and anthers enhance inter‐morph mating in comparison to intra‐morph mating and thus the maintenance of the polymorphism. In this study, we evaluate the relative importance of intra‐morph and inter‐morph pollen transfers in Narcissus assoanus, a species with dimorphic variation in style length but non‐reciprocity of anther positions. This stigma‐height dimorphism represents a transitional stage in theoretical models of the evolution of distyly. Seed set variation on recipient plants with donor plants of a single morph in experimental arrays in a natural population illustrate that inter‐morph cross‐pollination is more efficient that intra‐morph cross‐pollination as a result of high rates of pollen transfer from long‐styled to short‐styled plants. The observed rates of pollen transfer satisfy the theoretical conditions for the establishment of a stigma‐height dimorphism in an ancestral monomorphic long‐styled population in pollen‐limited situations. These results provide experimental evidence for the Darwinian hypothesis that enhanced inter‐morph cross‐pollination facilitates not only the maintenance of heterostyly but also the establishment of transitional forms implicated in the evolution of this polymorphism.     

Are pollinators the agents of selection on flower colour and size in irises?

Plant–pollinator interactions are believed to play a major role in the evolution of floral traits. Flower colour and flower size are important for attracting pollinators, directly influencing reproduction, and thus expected to be under pollinator‐mediated selection. Pollinator‐mediated selection is also proposed to play a role in maintaining flower colour polymorphism within populations. However, pigment concentrations, and thus flower colour, are also under selective pressures independent of pollinators. We quantified phenotypic pollinator‐mediated selection on flower colour and size in two colour polymorphic Iris species. Using female fitness, we estimated phenotypic selection on flower colour and size, and tested for pollinator‐mediated selection by comparing selection gradients between flowers open to natural pollination and supplementary pollinated flowers. In both species, we found evidence for pollen limitation, which set the base for pollinator‐mediated selection. In the colour dimorphic Iris lutescens, while pigment concentration and flower size were found to be under selection, this was independent of pollinators. For the polymorphic Iris pumila, pigment concentration is under selective pressure by pollinators, but only for one colour morph. Our results suggest that pollinators are not the main agents of selection on floral traits in these irises, as opposed to the accepted paradigm on floral evolution. This study provides an opposing example to the largely‐accepted theory that pollinators are the major agent of selection on floral traits.     

POLLEN-PISTIL INTERACTIONS IN TRISTYLOUS PONTEDERIA SAGITTATA (PONTEDERIACEAE). I. FLORAL HETEROMORPHISM AND STRUCTURAL FEATURES OF THE POLLEN TUBE PATHWAY

Heteromorphic characters and structural features of the pollen tube pathway are described in tristylous Pontederia sagittata to assess their influence on the pollination process and in mediating self-incompatibility behavior. Heteromorphic characters that distinguish the floral morphs include style length, stigma depth, stigmatic papillae length, stylar parenchyma cell length, area of the stylar canal, stamen height, anther size, and pollen size. Unlike several distylous species that have been investigated, the exine of pollen in P. sagittata was not strongly differentiated among the pollen types, and no differences in stigma cytochemistry were apparent. Features common to the floral morphs were a wet stigma, a hollow trilobed stylar canal separating into two sterile and one fertile canal, and a single anatropous ovule with a highly elaborated integumentary obturator. The similarity in structural features of the pollen tube pathway of P. sagittata to those found in monocotyledonous taxa with gametophytic self-incompatibility suggests that phylogenetic constraints may have influenced the evolution of trimorphic incompatibility in the Pontederiaceae.     

Ovarian self‐incompatibility in Narcissus papyraceus (Amaryllidaceae) is the result of a pre‐zygotic response

Ovarian self‐incompatibility, including pre‐ and post‐zygotic reactions, is a complex mechanism for which we still lack many details relating to its function and significance. The joint presence of ovarian self‐incompatibility with style polymorphism is a rare combination that is found in the genus Narcissus. Usually, style polymorphic species have heteromorphic (diallelic and linked to style length locus) incompatibility, which prevents fertilization between individuals of the same morph, thereby helping to maintain equal proportions of floral morphs in populations. However, when present, self‐incompatibility in Narcissus is not linked to style polymorphism and cross‐fertilization within each morph is possible. Hence, self‐incompatibility in Narcissus is of particular interest when attempting to unravel the nature of the rejection reaction and aiming to assess possible cryptic differences in the fertilization process in intra‐ and inter‐morph crosses, which might ultimately explain the wide variation of morph‐ratio in the field. We examined the breeding system of Narcissus papyraceus, a style‐dimorphic species that has biased morph ratios in most of its populations. We studied pollen‐tube growth in the pistil and ovule fate after experimentally controlled hand pollinations. The growth of pollen tubes in self‐ and intra‐ and inter‐morph crosses was similar up to the point of micropyle penetration in both morphs but, subsequently, a pre‐zygotic failure appeared to affect male and female gametophytes in selfed pistils. A high proportion of ovules from self‐pollinated flowers showed signs of collapse and self‐pollen tubes were blocked or behaved abnormally before entering the embryo sac. Self‐incompatibility was stronger in the long‐styled morph than in the short‐styled morph. We did not find any conclusive sign of differential functioning between intra‐ and inter‐morph cross‐pollinations in any morph. These results enable us to rule out the possible effects of pollen–pistil interactions in N. papyraceus as a cause of morph‐ratio biases and confirm the exceptional nature of the self‐incompatibility mechanism in this polymorphic species. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 629–643.     

Reproductive conflicts ofPalicourea padifolia (Rubiaceae) a distylous shrub of a tropical cloud forest in Mexico

We studied a population of the distylousPalicourea padifolia (Rubiaceae) in a cloud forest remnant near Xalapa City, Veracruz, México to explore possible asymmetries between floral morphs in the attractiveness to pollinators, seed dispersers, nectar robbers, floral parasites, and herbivores. We first assessed heterostyly and reciprocal herkogamy by measuring floral attributes such as corolla length (buds and open flowers), style and anther heights, stigma and stamen lengths and the distance between the anther tip to the stigma lobe. We then estimated floral and fruit attributes such as flower size, anther height, number and size of pollen grains, fruit size, seed size, nectar production, and flower and fruit standing crops to assess differences between floral morphs in attracting and effectively using mutualistic pollinators and seed dispersers. Also, floral parasitism and nectar robbing were assessed in this study as a measure of flower attractiveness to antagonists. The system seems to conform well to classical heterostyly (e.g. reciprocal stamen/style lengths, pollen and anther dimorphism, intramorph incompatibility) yet, there were several tantalizing differences observed between pin and thrum morphs. Thrum flowers have longer corollas and larger but fewer pollen grains than pin flowers. Both morphs produced the same total number of inflorescences, developed the same number of buds, and opened the same number of flowers per inflorescence during the flowering season. Nectar production and sugar concentration were similar between floral morphs but the reward was not offered symmetrically to floral visitors throughout the day. Nectar concentration was higher in pin flowers in the afternoon. The numbers of developing, fully developed, and ripe fruits were the same between floral morphs, however, fruits and seeds were larger than those of thrums. The incidence of fly larvae was higher among thrum flowers and damage by nectar robbing was the same between floral morphs. Fruit abortion patterns of flowers manually pollinated suggest intra-morph sterility (self and intramorph incompatibility). There were no differences between morphs in fruit and seed set per flower following legitimate pollination although thrums were more leaky than the pins (intramorph compatibility).     

Different rewards in female and male flowers can explain the evolution of sexual dimorphism in plants

Insects use floral signals to find rewards in flowers, transferring pollen in the process. In unisexual plants, the general view is that staminate (male) and pistillate (female) flowers obtain conspecific pollen transfers by advertising their rewards with similar floral signals. For female plants lacking food rewards, this can lead to floral mimicry and pollination by deceit. In this study, we challenge this view by presenting evidence for different rewards offered by flowers on females and males, as a mechanism promoting sexual dimorphism in Leucadendron xanthoconus (Proteaceae), a clearly sexually dimorphic shrub. The tiny beetle pollinators Pria cinerascens (Nitidulidae) depend entirely on the plants they pollinate for survival and reproduction. Male flowers provide mating and egglaying sites, and food for adults and larvae. Female flowers lack nectar and function to shelter pollinators from rain. Their flower heads have cup‐shaped display leaves, and are more closed than are those in males. On rainy days, flowers on females received 30% more visits than did flowers on males, and 90% more than they did on sunny days. When we removed display leaves in females, intact flower heads received 14 times more P. cinerascens visits than did manipulated flower heads, indicating that the cup shape attracts the beetles. In both sexes, having many flowers increased the probability of visits and the number of P. cinerascens visiting a plant. In males, the number of larvae was positively correlated with floral‐display size, while in females, seed set (pollen transfers) showed no relationship with floral‐display size. Ninety‐five per cent of the ovules received pollen and 52% matured into seeds. We explain the sexual dimorphism in L. xanthoconus as a result of an intimate partnership with P. cinerascens pollinators, in conjunction with a rainy climate. Pollinators favour large male floral displays, because they offer a reliable food source for adults and larvae. Frequent rains drive the P. cinerascens to leave males in search of the protection offered by females. Because females offer shelter, an essential resource that is not offered by male plants, they receive sufficient pollen independent of their floral‐display size. This pollination system promotes the evolution of sexually dimorphic floral signals, guiding pollinators to different rewards in male and female flowers. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85 , 97–109.     

Experimental tests of the function of mirror-image flowers

Enantiostyly, the reciprocal deflection of the style to the left or right side of the floral axis has evolved independently in at least a dozen angiosperm families. Unlike other plant sexual polymorphisms, the adaptive significance of these mirror‐image flowers remains unclear. Most authors have interpreted enantiostyly as a floral mechanism that promotes cross‐pollination. However, any functional interpretation is complicated by the fact that enantiostyly occurs in two forms. In monomorphic enantiostyly there are left‐ and right‐styled flowers on the same plant, while in dimorphic enantiostyly they are on separate individuals. In this paper we develop a model of pollen transfer which indicates that monomorphic enantiostyly should reduce geitonogamous pollination compared to a non‐enantiostylous condition, and that the lowest levels of geitonogamous pollination should occur in dimorphic enantiostyly. We tested these predictions using floral manipulations of bee‐pollinated Solanum rostratum in garden arrays. We compared mating patterns and fertility in five array types: non‐enantiostylous and straight‐styled, monomorphic enantiostylous, dimorphic enantiostylous, and arrays uniform for either left or right stylar deflection. Outcrossing rates in non‐enantiostylous arrays (t = 0.33 ± 0.04) were significantly lower than all other arrays, while monomorphic enantiostylous arrays (t = 0.74 ± 0.06) had significantly lower outcrossing rates than dimorphic enantiostylous arrays (t = 0.88 ± 0.04) and those uniform for stylar deflection (t = 0.84 ± 0.04). In dimorphic enantiostylous arrays, intermorph pollen transfer accounted for 75% of all outcrossing events. In pollen‐limited situations, both types of enantiostylous arrays had significantly higher female fertility than arrays fixed for one direction, demonstrating that enantiostyly promotes pollen transfer between flowers of opposite stylar orientation. Our results provide support for the hypothesis that enantiostyly functions to increase the precision of cross‐pollination in bee‐pollinated plants by reducing geitonogamy. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 85 , 167–179.     

Pre‐zygotic reproductive isolation between two synchronopatric Opuntia (Cactaceae) species in the Brazilian Chaco

• Opuntia (Cactaceae) is known for high rates of hybridization and ploidisation, resulting in the formation of new species. The occurrence of two sympatric and closely related species of Opuntia, O. elata and O. retrorsa, in Brazilian Chaco enabled us to test the hypothesis that pre‐zygotic reproductive isolation mechanisms operate in both species.
• We monitored the flowering period, as well as floral biology, and compared the morphological variation of floral structures through measurements, performed intra‐ and interspecific cross‐pollination tests, and recorded the guild of floral visitors and pollinators.
• Flowering was seasonal and highly synchronous. Floral biology exhibits similar strategies, and although floral morphology differs significantly in many of the compared structures, such morphological variation does not result in the selection of exclusive pollinators. Floral visitors and pollinators are oligolectic bees shared by both species. Opuntia elata and O. retrorsa are self‐compatible. While interspecific cross‐pollination (bidirectional) resulted in germination, the pollen tube did not penetrate the stigma.
• Opuntia elata and O. retrorsa are closely related; however, they are isolated and do not hybridise in Brazilian Chaco. We found that both have weak pre‐pollination barriers, but that they are strongly isolated by pollen–pistil incompatibility, i.e. post‐pollination barrier.

     

The role of pollinators in floral diversification in a clade of generalist flowers

Pollinator‐mediated evolutionary divergence has seldom been explored in generalist clades because it is assumed that pollinators in those clades exert weak and conflicting selection. We investigate whether pollinators shape floral diversification in a pollination generalist plant genus, Erysimum. Species from this genus have flowers that appeal to broad assemblages of pollinators. Nevertheless, we recently reported that it is possible to sort plant species into pollination niches varying in the quantitative composition of pollinators. We test here whether floral characters of Erysimum have evolved as a consequence of shifts among pollination niches. For this, we quantified the evolutionary lability of the floral traits and their phylogenetic association with pollination niches. As with pollination niches, Erysimum floral traits show weak phylogenetic signal. Moreover, floral shape and color are phylogenetically associated with pollination niche. In particular, plants belonging to a pollination niche dominated by long‐tongued large bees have lilac corollas with parallel petals. Further analyses suggest, however, that changes in color preceded changes in pollination niche. Pollinators seem to have driven the evolution of corolla shape, whereas the association between pollination niche and corolla color has probably arisen by lilac‐flowered Erysimum moving toward certain pollination niches for other adaptive reasons.     

Mixed hummingbird: Long‐proboscid‐fly pollination in ‘ornithophilous’Embothrium coccineum (Proteaceae) along a rainfall gradient in Patagonia,Argentina

Abstract The pollination ecology of eight populations of the tree Embothrium coccineum was studied along a steep rainfall gradient in NW Patagonia, Argentina. The showy red flowers suggest an ornithophilous pollination syndrome and they have been reported to attract hummingbirds in Argentina and hummingbirds and passerines in Chile. At each population, flower visitors were recorded and floral rewards were analysed. We found a highly significant increase in nectar concentration towards the drier end of the gradient, but this change was not related to the turnover of species in the flower‐visitor assemblage of E. coccineum. In addition to the hummingbird Sephanoides sephaniodes (Green‐Backed Firecrown, Trochilidae) which is widespread throughout the temperate forest at this latitude, other species seem to be locally important as pollinators of E. coccineum in some sites in Argentina (e.g. two long‐tongued tanglewing flies (Nemestrinidae) of the genus Trichophthalma). The long‐dated occurrence of tanglewing flies in South America, relative to the more modern hummingbirds, suggests that ornithophily may be a derived character in E. coccineum, the ancestral condition being pollination by Nemestrinidae.     

Evolutionary maintenance of stigma-height dimorphism in Narcissus papyraceus (Amaryllidaceae)

Stigma-height dimorphism is a sexual polymorphism in which plant populations are composed of two floral morphs that differ significantly in style length but not anther position. The morphs exhibit approach and reverse herkogamy, floral designs that in most species typically occur as monomorphic conditions. We investigated the floral biology of stigma-height dimorphism in the Mediterranean geophyte Narcissus papyraceus (Amaryllidaceae) in an effort to understand the evolutionary forces maintaining stylar polymorphism. Our survey of 66 populations in Spain, Portugal, and Morocco indicated that 56% were dimorphic with the long-styled morph at an average frequency of 0.79. The remaining 44% of populations sampled were monomorphic for the long-styled morph. In dimorphic populations there was a significant positive relation between population size and the frequency of the short-styled morph. Controlled pollinations demonstrated that N. papyraceus is self-sterile with no significant differences in female fertility between intra- and intermorph crosses. Prior self-pollination reduced seed set in flowers that were subsequently cross-pollinated. Estimates of mating patterns using allozyme markers in eight populations indicated that N. papyraceus is largely outcrossing (mean t(m) = 0.81) with no significant differences between monomorphic and dimorphic populations or style morphs. Stigma-height dimorphism in N. papyraceus is maintained in populations by insect-mediated cross-pollination with biased morph ratios and stylar monomorphism likely resulting from the combined influence of the inheritance of the polymorphism, morph-specific differences in assortative mating and founder effects.